Data communication typically occurs as the transfer of information from one communication device to another. This is typically accomplished by the use of a modem located at each communication endpoint. In the past, the term modem denoted a piece of communication apparatus that performed a modulation and demodulation function, hence the term “modem.” Today, the term modem is typically used to denote any piece of communication apparatus that enables the transfer of data and voice information from one location to another. For example, modem communication systems use many different technologies to perform the transfer of information from one location to another. Digital subscriber line (DSL) technology is one vehicle for such transfer of information. DSL technology uses the widely available subscriber loop, the copper wire pair that extends from a telephone company central office to a residential location, over which communication services, including the exchange of voice and data, may be provisioned. DSL devices can be referred to as modems, or, more accurately, transceivers, which connect the telephone company central office to the user, or remote location, typically referred to as the customer premises (CP). Various DSL communication devices use different formats and different types of modulation schemes and achieve widely varying communication rates. However, even the slowest DSL communications devices achieve data rates far in excess of conventional point-to-point modems.
Some of the available modulation schemes include pulse amplitude modulation (PAM), quadrature-amplitude modulation (QAM), and carrierless amplitude/phase (CAP). In a PAM communication system, information in the form of an n-bit word is encoded to a number of voltage levels. The voltage levels are selected so that the encoded bits can be decoded at a receiving device. This process is sometimes referred to as “mapping.” The PAM voltage levels correspond to the bits of information to be transmitted and the bits are encoded into a signal constellation. The transmit signal is transmitted to a receiving device. The receiving device analyzes the received waveform and attempts to decode the waveform to recover the encoded bits.
In a conventional PAM transmitter, as defined in the ITU-T draft recommendation G.991.2 (G.shds1), incorporated herein by reference, a PAM system having 16 levels is defined. Alternatively, 2K levels, in which K represents the number of encoded bits and is an integer, are possible. Changing the number of encoded bits (K) results in undesirably large (on the order of 6 dB) performance degradation. Such a large performance degradation indicates that the PAM transmitter may be operating up to 6 dB below channel capacity.
Thus, it would be desirable to have a PAM communication system capable of using most or all of the available channel capacity.